Genetics of Daphnia obtusa: Genetic Load and Linkage Analysis in a Cyclical Parthenogen

Inbred and outbred progeny were used to assess the presence of genetic load among clones of the cyclically parthenogenetic cladoceran Daphnia obtuse. The average proportion surviving was significantly lower (0.43 ± SE 0.05) for progeny from 18 self-mated clones than for progeny from five outcrossed matings (0.88 ± 0.06). The variation in survival among self-mated clones (0.13 to 0.92) suggested that genetic load varies among clones and may be related to the age of each clone. Inbred progeny from two clones exhibited slower growth and an increased variation in growth compared to outbred progeny from the cross between the two clones. Genotyplc segregation ratios were tested in the selfed progeny of 14 clones heterozygous for one or more enzyme loci. Seven (25%) of the 28 tests at individual loci showed significant deviations from the expected 1:2:1 ratio among the three genotypes. Deviations were caused by a deficiency of one of the homozygous classes, suggesting the presence of linked recessive deleterious genes. Information on recombination among three enzyme loci revealed two linkage groups consisting of Pgm-Amy (recombination fraction 0.15 ± 0.04) and Got. Recombination was found to occur in both sexes.